U.S. Pat. No. 3,993,924 to Hanf discloses a traveling wave tube having a delay line comprised of axially aligned rings supported by members which extend alternately from two facing sides of the waveguide inner walls. The rings are not connected by any axially aligned bars.
U.S. Pat. No. 3,443,146 to Buck discloses a traveling wave tube delay structure comprising a rectangular waveguide having stubs extending inwardly, alternately from a pair of opposing walls. Each stub is provided with an aperture, the apertures being coaxial with the longitudinal center of the waveguide. The metal surrounding each aperture serves as a ring. Longitudinally extending bars interconnect the rings with each bar being at a position on the ring, which position is 180.degree. away from the position of the other bar connected to the ring.
U.S. Pat. No. 4,066,927 to Gross discloses a delay line for a traveling wave tube, particularly for use with millimeter waves. Elongated attenuating members are disposed in the respective cells defined by transverse walls. The attenuating members are matched by a suitable adjustment of matching cylinders or pins in the respective immediate adjacent line cells. The Gross patent does not disclose a conductive path in the direction of wave propagation.
U.S. Pat. No. 3,335,314 to Espinosa et al discloses a ring bar type slow wave circuit. Espinosa shows thick stubs extending perpendicularly from the walls of a waveguide to support rings. The currents in the respective stubs are predominantly transversal or perpendicular to the axis.